1. Field of the Invention
This invention relates to a reduction smelter or smelting furnace for iron and non-iron metals, and more particularly to a gasification burner for powdered solid fuel which serves as the source of reducing gas and heat for a zinc reduction smelter or smelting furnace.
2. Description of the Prior Art
In the most common Imperial Smelting Process (ISP) method of pyrometallurgical zinc smelting, zinc sulfide concentrate is roasted, and the lump sinter obtained through roasting is put into the smelting furnace together with the lump coke. The zinc is evaporated out, so that a reducing gas containing the zinc vapor is obtained. The zinc vapor in this gas is absorbed by a lead splash condenser, and recovered to obtain crude zinc.
In comparison to other pyrometallurgical smelting methods, this ISP method has an advantage that lead and zinc can be processed at the same time, thus being very cost competitive.
However, recently due to very intense cost competition, the cost competitiveness of this ISP method is very inefficient. The reason is the high cost of using lump coke.
On the other hand, in countries like Japan where the cost of electricity is high, hydrometallurgical smelting is also unfavorable.
Therefore, in order to improve the cost competiveness of the ISP method many improvements to the pyrometallurgical method of smelting have been proposed and researched.
One of the most promising of these proposals has been disclosed in Japanese Patent Publication No. Showa 61-28004, "Injection Smelting of Zinc Calcine". In this method, slag which has a Fe/SiO.sub.2 ratio close to that of the zinc concentrate, and a crude lead metal, are contained beforehand in the furnace, and the roasted powdered zinc concentrate, a reducing agent, and highly oxygenenriched air are blown through a lance into the melt bath. Powdered coke and/or fine carbon material is used as the reducing agent. When compared to the ISP method, because powdered coke and/or fine carbon material is used in the place of lump coke, it is possible to greatly reduce the operating expenses.
However, in this method, because the retention time of the powdered coke in the furnace is very short, the effective utilization of the powdered coke, or in other words, the gasification rate of how much of the carbon burns and gasifies to gas, is much worse than the ISP method regardless of how small the grain diameter of the powdered coke is compared to lump coke. As a result, a large amount of the unburned powdered coke is scattered on the post-process condenser, hindering condensation of the zinc. Therefore the rate of recovery for zinc is made lower.
Two inventions have been proposed to do away with this problem. They are disclosed in Japanese Patent First Publications No. Sho 62-80234 and No. Hei 1-129933. These two inventions are both related to the blowing-in lance method.
The invention described in Japanese Patent First Publication No. Sho 62-80234 uses a double pipe construction for the lance having outer and inner pipes. Fine coke is supplied through the inner pipe, and oxygen-bearing gas such as oxygen per se or oxygen-enriched air is supplied through the outer pipe, and the fine coke and the oxygen-bearing gas are mixed in the mixing section on the tip of the lance.
The invention described in Japanese Patent First Publication No. Hei 1-129933 uses a zinc calcine supply nozzle located in the center, and several supply nozzles for mixing and discharging the fine coke and oxygen-bearing gas located around the central zinc calcine supply nozzle.
Both of these inventions are characterized by the mixing section being located in the narrow portion of the nozzle tip section where the fine coke and oxygen-bearing gas are mixed, and then after mixing, the mixture is discharged into the furnace from the nozzle outlet port at nearly the speed of sound.
Use of the lance as described in both of the aforementioned inventions improves the gasification rate of the fine coke a degree. However as time elapses, they both experience a rapid decline in the gasification rate, and also the zinc recovery rate declines very rapidly. The reason for this is that the discharge velocity is so high, that the tip of the burner is quickly worn away by the fine coke, and thus the mixture condition of fine coke and oxygen-bearing gas becomes poor causing a decrease in gasification rate and combustibility.
Also, the fine coke which is blown using a lance as in these two inventions, is gasified in an oxygen-poor state inside the furnace. Because the furnace temperature is kept below 1500.degree. C. in order to protect the bricks of the furnace, there is a limit to how far the gasification rate of the fine coke can be improved, regardless of how well the fine coke and oxygen-bearing gas are mixed, for the following reason.
As is well known, in an oxygen-poor state, carbon, which is the main component of the fine coke, is gasified according to the following equations. EQU C(s)+O.sub.2 (g)=CO.sub.2 (g) Eq. 6 EQU C(s)+CO.sub.2 (g)=2CO(g) Eq. 7
In other words, oxygen first reacts with carbon, as shown in Equation 6, to form CO.sub.2 depending on the amount of oxygen. Next, this CO.sub.2 reacts with the remaining carbon, as shown in the Equation 7, to form CO. As is well known, the reaction of Equation 6 is an exothermic reaction which advances very quickly. However the reaction of Equation 7 is an endothermic reaction, and its rate of reaction has a positive correlation with temperature. At temperatures around 1500.degree. C., the rate of reaction of Equation 7 is comparatively slow, and in order to convert all of the remaining carbon into CO, the carbon must remain in the furnace for a long time. However, when the two inventions mentioned above are used, the period of time that the carbon remains in the furnace cannot be lengthened.